It is known that latent electrostatic images can be developed with toner particles dispersed in insulating or non-polar liquids. Such toner particles normally comprise colouring matter such as pigments which have been ground with or otherwise combined with dispersing resins or varnishes or the like. Such toner particles usually have combined therewith fixing materials which are commonly but not necessarily thermoplastic polymeric materials. Additionally polarity control or charge directing agents are usually included to control the polarity and charge mass ratio of the toner particles. Such dispersed materials are known as liquid toners or liquid developers.
Initially liquid developers were used commercially in office copiers of the non-transfer type, that is in which the electrostatic latent image on a zinc oxide/resin binder photoconductive coating on a paper sheet was developed thereon and fixed thereto. The toner particles were not required to be transferable to other receptor surfaces. Subsequently when it was desired to transfer such toner deposits to a receptor surface using electrostatic transfer techniques problems were found relating to transfer efficiency, image shift, bleed and loss of resolution, well known to those skilled in the art.
The earliest liquid toner compositions known to us to have been formulated with a view to overcoming transfer problems previously referred to are as revealed by Wright in U.S. Pat. No. 3,419,411, now expired. In this disclosure so called lattice forming materials were included in the toner compositions to inhibit unwanted shift of colouring matter during image transfer. Such lattice forming materials were considered to form a fibrous matrix within the toner deposit whereby the colouring matter was restrained in place within such
image deposit and enabled to be transferred truly without lateral shift to a receiving sheet using conventional electrostatic transfer techniques. Such lattice forming materials were also considered to maintain the image deposit in a condition amenable to transfer whereby transfer efficiencies in excess of ninety per cent were readily obtained. Further as dispersant liquid contained within the image deposit was removed after transfer such image deposit became mechanically fixed to the receptor sheet and did not back transfer if subsequent imaged deposits were transferred thereon. The lattice forming materials disclosed were rubber modified polystyrene, paraffin wax and ethyl cellulose. Ethyl cellulose was used in conjunction with a thermoplastic binder material such as polyisobutyl methacrylate, whereas paraffin wax was used in conjunction with a polymerised linseed oil/calcium resinate varnish.
Landa in U.S. Patent No 4,842,974 particularly argues against the Wright disclosure with regards to the lattice forming properties of ethyl cellulose or rubber modified polystyrene. His argument against paraffin wax is less specific and in fact admits that control of the KB value of the non-polar liquid dispersant affects the absolute resolution capabilities of the liquid toner, which could well be an admission of the validity of the Wright argument. However Landa describes and illustrates plasticising a thermoplastic polymer and a pigment with a non-polar liquid at an elevated temperature, cooling to form a sponge and further grinding such sponge with additional non-polar liquid to pull the particles apart and form fibres thereon and extending therefrom, which could be described as a mechanical equivalent to the lattice of Wright.
Mitchell, in U.S. Patent No 4,631,244 also discloses the formulation of liquid toners containing resinous "fibres" which term is stated to mean pigmented toner particles formed until fibres, tendrils, tentacles, threadlets, fibrils, ligaments, hairs, bristles or the like. Thermoplastic polymers are
disclosed, specifically acrylic polymers and co-polymers and ethylene vinyl acetate co-polymers. Temperature controlled attritor milling is used to obtain toner particles of the required particle size range. In addition to the non-polar dispersant liquid, other liquids of higher KB value are included during the grinding step, which is also in part anticipated by Wright. Control agents are also included, but generally other aids to dispersion such as oils or varnishes are omitted, as also are lattice or fibre forming materials such as waxes.
The present invention uses many of the materials of the prior art and in addition uses known milling methods for toner preparation, but in view of the prior art the resultant liquid toners exhibit some surprising features as will become apparent in the following.